Synthetic aperture (SA) imaging can be used to increase resolution beyond the diffraction limit of a physical aperture of an imaging system. In SA imaging systems, a large “virtual” aperture is synthesized by illuminating a target region with electromagnetic signals transmitted from a moving platform and collecting phase-coherent return echoes produced by reflection of the electromagnetic signals from the target region. The return echoes are recorded and then coherently combined using signal processing techniques to reconstruct a high-resolution image of the target region. SA imaging was initially developed and has been successfully employed at radio frequencies, where it is referred to as “synthetic aperture radar” (SAR). Conventional SAR systems typically operate in the centimeter (cm) wavelength range and produce images with azimuth resolutions of the order of a decimeter (dm) to a meter (m). As resolution is generally inversely proportional to the imaging wavelength, there has been a growing interest to extend SAR to shorter wavelengths. In this context, an emerging technology referred to as “synthetic aperture ladar” (SAL) has been developed to extend SAR to visible and near-infrared frequencies.
SA imaging systems provide two-dimensional (2D) SA images representing projected ground surface reflectance. A 2D SA image can be represented as a two-dimensional complex-valued array of pixels, where each pixel has an amplitude value and a phase value. The two dimensions of the 2D SA image are the azimuth and the slant-range directions. For a target region having a non-flat topography, an ambiguity exists between ground range and height since various pairs of ground-range and height values may lead to a same slant-range value.
An approach to remove this ambiguity and provide three-dimensional (3D) imaging of a target region is known as “interferometric SA imaging”, referred to as IFSAR and IFSAL depending on the operating wavelength. In this technique, two 2D SA images are acquired from different points of view relative to the target region. The 2D SA images are co-registered and interfered with each other, and an elevation map of the target region is extracted from their phase difference. A challenge in implementing interferometric SA imaging is that the height reconstruction process involves phase unwrapping, which can suffer from robustness limitations. This is especially true in the case of IFSAL, since the conditions on phase accuracy and platform stability required for interferometry become increasingly stringent as the wavelength decreases. Another challenge is that since a 2D SA image involves the projection of a 3D target region onto a 2D image plane, slant-range distortion effects such as foreshortening and layover can appear for target regions with irregular topography.
Laser-based scanning techniques such as scanning lidar provide another approach to achieving 3D imaging of a target region. These techniques can be implemented using various distance measurement methods, including time-of-flight, phase-shift, and frequency modulation methods. However, although laser-based scanning techniques can provide 3D images, their spatial resolution is limited by the size of the beam illuminating the target region.